Method and apparatus for determining the resonant frequencies of networks



I Nov. 25,- 1947. w. A. McCOOL METHOD AND APPARATUS FOR DETERMINING THERESONANT FREQUENCY OF NETWORKS Fiied July 6, 1944 gwue/wtom I WILLlAM A.MQCOOL Patented Nov. 25, 1947 mn i non mussel-armres .ING JIHE R'ESONANTAEREQUENCIES or 1 NETWORKS ".William AflVlcCool, Washington, D. mplicationnuiyie, 1944;Sei-3QFND53435148 1'5 "Claims.

' (-'Gra'rited underthe act of March 3, 1883, as

amehiiGH'ADfil-BO, 1928; 370 O. G. 757) This invention relates to thedeternlinationbf resonant frequencies 'of"reactive-networks "and isparticularly directed to the problem of ascertainin'g the f resonantfrequency "of transmission lines'ections.

It is accordingly the object of the invention to-provide a "novel methodand means ofascertaming the-resonant frequencies dfrea'ctivemet- Works.

.A further' object "of the zinventionis to deterniinetlie velocity of'propagationfof a wave "along a line section. This is accomp'iiShedbyascertainin'g the' resonatingfrequency for an unterminated line sectionof known 'length. At such frequency the line section comprises an coldIt quarter-wave multiple of the wave length propagated therein. Inasmuchas tl-i'e' order of' magnitude of the velocity-is;ordinarily known fromthe construction of the cable, the particular multiplefii ay beobtained. Consequently, from -the value of the --excitin frequency, "thevelocity of propagatio'nmaylee-calculated. I

'i i n exemplary form of apparatus suitable for elfectu'ating the methodof the presentk'invention is ishown in the-drawing, in connecti on withwhich the invention will be further described.

The system of the present invention includes a shielding cabinet 1subdivided at 2 into two compartments 3 and 4. Compartment 3 contains avariable frequency oscillator including a triode 5 coupledconventionally with a tuned circuit including inductance 6 and tuningcapacitor 1. Balanced capacitor 8 is provided for trimming adjustment ofthe oscillator frequency.

The oscillator inductance is Very loosely coupled through shield 2 withinductance I0, which forms a tuned circuit with tuning capacitor II andinductance l2. Trimming capacitor I3 is also provided.

The desired coupling of inductances 6 and I0 may be established byemploying a sheet metal shielding member 2 suitably apertured. Undersuch loose coupling the coil l0 may be considered as driven by aconstant series voltage.

The coupled circuit includes in series a jack M for receiving connectionof the network Whose resonant frequency is to be determined. In thedrawing an unterminated concentric line section is shown, which isresonant at frequencies at which it constitutes a quarter wave length oran "odd'r'iiiiiltifile lthere'o'f. LAt ,resonant ..-'frequenciesitheiifiipedan'ce appearing acrossjack l.4.wil1iconstitiifte a more I"ista'nc'e which will not afiect the .r souautsrrequenc Lthe :tu'neuicir'c'uit.

As "showhfih'fithe Idra" mg, the Itarik -condense'r l and ithe condenser"I I .6f "the tuned circuit are igangedfltoa lcommonfcoiitrol 1J5.Y'TEor lining .up the. ci'flillits, Taj S6113. ibohdubtoi :maybeihse'rteli tofshfiftjjaik f4 anuftrimme'rsfaana 13 adjusted YSO thattheilfitll'fal frfidlfallit freque cies 6f "the oscillatortank circuitand the *coupled circuit substantially coincide throughout the tuningrange. This may be "determined "by a vacuum itubeivoltinter lb "of. any"suitable. type connected, "forinstance; t0 "11 T2. OhJ'cdllfittra'kin'g 'the foscillatori'arid coupled circuit are "tuned-tdsiiher throiighflie rang "and voltmeter *1 6 will indicate"afsubstantially uniform voitagefthroughout. (on connection"'of'jthemetwork "to." jack f4, the ireactanoa'thereof willdetunetheicoupled "circuit so -fthat no voltnieter i'ndication"isobtairfed-*-except there'sonaiiEfrequency of thenetwork. Conseq may fthelatter mfaybeaite'rmined immedi- "any adjusting metered l-i-for'ama'inmum deflection of voltmter -Fli.

For obtaining the velocity of propagation within a cable section, theapparatus may be employed to yield direct results by suitablecalibration of control l5. Assuming normal construction wherein theaverage dielectric constant within cables of the type considered is2.25, and that measurements are to be taken at about mc./s., the wavelength within the cable will be substantially 200 cm., and a threequarter wave section is cm. long. If, therefore, a 150 cm.

section cable be inserted in the jack l4, its resonant frequencyadjacent 100 mc./s. as determined by control l5 corresponds to threequarter wave lengths. Since the period required for propagation to theend of the line is %.1/f seconds, the velocity in centimeters per secondis 200 f. The velocity expressed in per cent of the velocity in freespace is /3 f where f is expressed in megacycles. Control l5 may bedirectly calibrated in the velocity of propagation, if desired.

Where a number of cable sections are bein tested, the reading of vacuumtube voltmeter I6 varies inversely with their attenuation and willafford a rough indication of the relative properties of the lines inthis respect.

Where the constructional characteristics of a line are unknown andcannot conveniently be determined, the quarter wave length multiple mayi be found by ascertaining two adjacent resonant frequencies of thesection, Since the wave length is inversely proportional to thefrequencies, and only the ratio of the former is required, the desiredinformation may be derived from the formula where A is the shorter ofthe adjacent resonant wave lengths.

, line section.

It will be understood that the other line lengths may be employed andthat theillustrations discussed are exemplary only. xItis'furtherapparent that a short-circuited half wave or multiple half wave sectionmay be employed.

The invention described herein may b'e'm anu 1 factured and used by orfor the Government of the United States of America for governmentalpurposes without the payment of anyroyalties thereon or therefor.

Iclaim:

1. Means for ascertaining the resonant frequency of an electricalnetwork including an oscillator tunable over a range of frequencies, acircuit tunable over saidrange of frequencies coupled to saidoscillator, common control means for simultaneously tuning theoscillator and the coupled circuit togetherthrough said range intracking coincidence, means for indicating the response developed in thecoupled circuit, and means for electrically coupling a reactiveelectrical network with said coupled circuit to detune the latter exceptat'the resonant frequency of the network.

2. Means for ascertaining the resonant frequency of an electricalnetwork including an oscillator tunable over a range of frequencies, a

circuit tunable over said range? of frequencies coupled to saidoscillator, commoncontrol means for simultaneously tuning the oscillatorand the coupled circuit together through said range in trackingcoincidence, means for indicating the:

response developed inthe coupled circuit, and means for connecting anelectrical network in series in said coupled circuit whereby the latteris detuned and unresponsive except at the resonant frequency of thenetwork.

4. Means for ascertaining the resonant frequency of an electricalnetwork including an oscillator circuit tunable over a range offrequencies, a circuit tunable over said range of frequencies coupled tosaid oscillator, means for simultaneously tuning the natural frequenciesof the oscillator circuit and the coupled circuit over said. range intracking coincidence, means for electrically coupling a reactiveelectrical network with one of said circuits to detune said circuit,except at the resonant frequency of the network,

I reactive network.

2 Number and means for indicating the signal level in the I coupledcircuit.

5. The method of ascertaining the resonant frequency of a reactivenetwork electrically coupled to a resonant circuit 'excitedby a tunableoscillator comprising the steps of tuningv the oscillator over a rangeof frequencies, simultaneously tuning the resonant circuit over the samerange of frequencies in tracking coincidence with. the

oscillator, and indicating the signal level in the resonant circuit todetermine the frequency at which the resonant circuit is nct detuned bythe WILLIAM M cooL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Name Date Y Schiepe Aug. 24, 1937 Schrader June20, 1939 Broaten Oct. 3,1939 Dennis et a1. May 25,1943 Mittelmann 'July20, 1943

